Uptake and distribution
The path and extent of distribution on the plant surface and uptake into plant tissues are characteristics that allow for a differentiation among fungicidal active substances. Non-systemic active substances do not penetrate into plant tissues, and are therefore unable to reach fungal structures that have already developed within the plant. So these active substances can only be applied to obtain protective activity. Fungicides with systemic properties can be applied after the pathogen has succeeded in penetrating into the plant’s tissues, as the internal transport of the active substance allows it to reach the fungal structures in order to kill them off. However, curative activity is only possible up to a certain point.

What are the characteristics?
Contact fungicides lack systemic activity, but they usually possess a very broad spectrum-of-action. They are unable to penetrate through the plant cuticle, so they cannot act within the plant’s tissues. The converse of their broad spectrum of action is limited selectivity. The user must therefore take a number of drawbacks into account:

• Only the initial developmental stages of the pathogen are affected. As soon as the fungus succeeds in penetrating into the plant, it can continue to develop uninhibited.
• The fungicidal spray deposit must form an even cover over the plant surface in order to guarantee adequate protection.
• Plant tissues that develop after the treatment has been applied remain unprotected. It may therefore be necessary to apply these active substances several times in succession, depending on the prevailing weather conditions and the stage of growth of the crop.

Mesostemic fungicides are active substances that are taken up extensively at the plant surface. They tend to form a depot of active substance from which a continuous transfer takes place, either into the plant, or across its surface. The result is a much-extended duration of activity.

Systemic active substances also succeed in penetrating through the plant’s cuticle, so they can express their activity within its tissues. They are usually selective, and can be used both protectively and curatively. The spectrum of action of these fungicides is commonly restricted to a group of related fungi, although the degree of selectivity differs among the various active substances. The selectivity contributes to the environmental safety of the substances: it also allows a more targeted use of products against pathogens that are already present in a crop.

Resistance management
Only a few groups of active substances are available for controlling the most important fungal diseases of cereal crops. The repeated use of fungicides from the same class of active substance encourages the development of resistance, which can quickly become a fixed genetic feature, spreading rapidly within the fungal population. Eventually, the resistant variant of the pathogen predominates, and all fungicides with the same mode-of-action are equally useless against it.

In order to prevent the development of resistance to fungicides, spraying programmes should include active substances with different modes of action. Unnecessary applications and treatments with reduced application rates should also be avoided – these not only increase the risk of resistance development, they also represent a false economy if they fail to check the progress of the disease.